The invention relates to a device for modulated optical signals for coherent optical generation of modulated phase-controlled RF charge carriers and for summing such signals on a plurality of optical carriers.
Optical devices of this kind have been disclosed by the following publications: Wale, M. J. and Birkmayer, W. S. "Coherent optical techniques in antenna beamforming for satellite communications," Colloquium on Optical Intersatellite Links and Onboard Techniques, IEEE, London, Jan. 12, 1990 (IEE Digest 1990/8), as well as Birkmayer, W. S. and Wale, M. J. "Optical BFN for telecom satellites and/or SAR applications; analysis and results," in Proc. ESA Workshop on Advanced Beamforming networks for Space Applications, November 1991, ESA publication WPP-030. FIG. 3 shows the block diagram of an example of an optical beamforming network in which the optical device is incorporated into a combiner. In the example shown, an optical carrier is superimposed with two modulated optical carriers. In particular, the publications discuss the problem that occurs in conjunction with the use of coherent optical methods to generate and control several modulated RF carriers, e.g., in producing antenna lobes in phase-controlled antennas.
For the arrangement with an RF carrier, the required RF signal is obtained by mixing two optical carriers with frequencies w.omega..sub.1, .omega..sub.L0, separated by the required superimposition frequency .omega..sub.RF. Since any phase or amplitude encoding on .omega..sub.1 or .omega..sub.L0 is transferred by the mixing process in the optical receiver directly to the RF carrier with a frequency .omega..sub.RF, phase or amplitude control can be used in the optical range to control the corresponding properties of the RF signal. This is the source of the advantages in the described system presented in the above publications. The basic configuration, as used to control phase and amplitude in several phase-controlled antenna elements, is shown in FIG. 3. Here the two optical carriers with frequencies .omega..sub.1, .omega..sub.L0 are generated by a pair of lasers operating under the control of an electronic monitoring circuit (PLL). The latter monitors the relative frequency and/or the phase of the RF carrier and transmits a feedback signal to one or both lasers, so that a constant frequency ratio, in some cases a constant phase ratio as well, is maintained between the measured RF carrier and a reference frequency from an external control oscillator.
The above-mentioned publications describe an embodiment which is concerned primarily with providing an optical output for the measurement process, with this output having certain properties. In order to permit accurate RF phase control, the temperature coefficient of the phase of the differential frequency applied to the output must be as low as possible and the control signal must travel a path other than the one followed by the antenna signals which are controlled independently with the smallest possible expense. The signal applied to the monitoring output must not be highly modulated, since data modulation sidebands interfere with the operation of the feedback control circuit of the laser for phase/frequency and result in coupling between data modulation and the controlled RF frequency or phase. In the special case of a modulation circuit with suppressed carrier, such as QPSK, the carrier is not present in the spectrum of the modulated signal at all and must be regenerated in the laser control circuit, for example by nonlinear signal processing. Consequently, however, the electronics must be designed in a very costly fashion, despite which the accuracy and temperature stability of the system can be considerably adversely affected.
The goal of the present invention is to provide an optical device of the species recited at the outset in which the previous cost of specially designed optical and mechanical components and the consumption of electrical power are considerably reduced and the signal/noise ratio of the feedback signal is drastically improved.
This object is achieved according to the invention by providing a pure carrier output without modulation; for this purpose the optical signals of two orthogonal polarization states are processed separately, with the optical processing functions being performable using conventional prisms, polarization beamsplitters, and polarizers or by means of integrated optics. (The latter is particularly advantageous as far as power, sturdiness, and stability against environmental factors are concerned.) One of the two orthogonal polarization states is used to transmit the optical signal in an unmodulated state and the other serves to transmit the modulated optical carrier. This can be accomplished by using an integrated optical device, a phase modulator for example, which modulates only one polarization direction. For this purpose a device is used which is based on the electro-optical effect in semiconductor materials of the main groups III-V such as GaAs/AlGaAs, InP/LnGaAs, or InP/GaAsP. By using a vertical electrical scattering field in a hollow conductor, manufactured for example on a wafer with orientation &lt;001&gt;, the TE-polarized wave is modulated by the electro-optical effect .tau..sub.41 while the TM wave undergoes no electro-optical modulation whatsoever. It should be mentioned in this connection that other modulation effects that influence the TM mode, which result for example from the movement of charge carriers, can be kept low by selecting suitable parameters in designing the device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.